As being highly tough, transparent and moisture-proof, films of crystalline propylene-based polymers are widely used for wrapping and packaging films. For their use, wrapping and packaging films are often formed into bags. In an ordinary process that comprises forming a film into bags, filling the bags with objects, and sealing the open end of the thus-filled bags, two film edges are heat-sealed by pressing them against a hot rod. In the recent process of forming film bags or wrapping or packaging objects with films, used are large-scale high-speed wrapping machines for improving the productivity. For these, it is much desired to develop materials with good heat-sealability. Wrapping and packaging films must be smoothly worked into secondary products, and are therefore desired to have good slip characteristics and anti-blocking characteristics as their indispensable properties.
However, films of propylene homopolymers are defective in that, when heat-sealed, they require high temperatures and take a lot of time. To solve this problem, a technique of copolymerizing propylene with ethylene, 1-butene and any other .alpha.-olefins is widely employed.
However, in the prior art technique of producing propylene-based polymers in the presence of a Ziegler-Natta catalyst, a large amount of comonomers including ethylene, A 1-butene and other .alpha.-olefins must be copolymerized with propylene in order that the resulting copolymers could have satisfactory heat-sealability. These comonomers often remain in the copolymers, essentially forming a low-molecular-weight fraction therein, and the fraction has a low degree of crystallinity and makes the copolymers sticky (the fraction is hereinafter referred to as a sticky fraction). As a result, films of the copolymers lose the advantage of toughness that is intrinsic to polypropylene films, and, in addition, their surfaces are often blocked to detract from the secondary workability of the films. What is more, the comonomers remaining in the copolymers will often bleed out to whiten the surfaces of the copolymer films. For these reasons, the copolymers are not on the practicable level.
To solve the problem with the prior art technique, tried is dissolving the sticky fraction in an inert solvent for removing it. However, it is extremely difficult to effectively wash out the sticky fraction while the low-temperature-melting fraction that contributes to the heat-sealability of the copolymer films is prevented from being washed away along with the sticky fraction. Therefore, the method of dissolving the sticky fraction could not produce satisfactory results in industrial applications.
On the other hand, various studies are being made these days for using metallocene catalysts in production of propylene-based polymers. Polymers produced in the presence of metallocene catalysts are characterized in that they have extremely narrowed molecular weight distribution and compositional distribution. However, for ensuring molding stability of polymers and for making polymer moldings have various physical properties that will be contradictory to each other, it is said that polymers having broad molecular weight distribution and compositional distribution are rather better. Taking it into consideration, various compositions of propylene-based polymers produced in the presence of metallocene catalysts have been proposed. Until now, however, no one has succeeded in realizing a satisfactory industrial-scale method of freely producing resin polymers that have desired properties and are suitable to intended applications and molding methods.